I do get an odd popup, which disappears after a few seconds...
You will see that if you log into the webgui as soon as it becomes available but before bootup has completed. As you are seeing it should disappear when bootup completes.
Yes you will need to reinstall clean because that is the last 32bit version. That hardware supports 64bit though and will run the current version. It's not the fastest but if it's working currently it should still be good. It will pass 250Mbps easily.
You should be able to backup the config, reinstall 2.7.2, then restore the config into it.